The explosive growth of the optical communication traffic in recent years, driven by bandwidth hungry applications and progress in transmission technologies alike, has greatly contributed to ever-increasing demand for higher capacity optical networks offering more flexibility at lower cost. Two different yet complimentary trends are becoming more and more evident in this respect. First, deep penetration of the optical fiber into the access networks and, second, greater demand for capacity, bandwidth provisioning and agility back up into the upper layer networks. Both require massive deployment of the optical gear that drives the traffic along the fiber links, on a scale not seen in earlier generation networks. Specifically, optical transceivers, which receive downstream and send upstream data signals, have to be deployed at every optical line terminal or/and network user interface in the access optical networks, but they also are the key optical components to be installed at every node of opaque (local or metropolitan area) optical networks. Whereas performance requirements, e.g. in terms of speed, optical power or sensitivity, for such optical components may be relaxed as compared to their upper layer networks counterparts, cost efficiency and volume scalability in manufacturing are increasingly becoming the major requirements for their mass production.
Photonic integrated circuits (PICs), in which multiple elements of common or different functionalities are monolithically integrated onto one chip, are an attractive solution to mass production of highly functional optical components in that they enable scalable volume manufacturing by means of semiconductor wafer fabrication techniques. As such PICs offer the ability to dramatically reduce the component footprint, avoid multiple packaging issues, eliminate multiple optical alignments and, eventually, create economical conditions in which optical components achieve the cost efficiency and volume scalability enabling to transfer them into consumer photonics products. The advantages of PIC technology become especially compelling when active waveguide devices, such as laser or photodetector, are combined with the passive waveguide circuitry to form a highly functional photonic system on the chip. Since the active devices usually all are made from artificially grown semiconductors having bandgap structure adjusted to the function and wavelength range of their particular application, such semiconductors are the natural choice for the base material of the PICs. For example, indium phosphide (InP) and related III-V semiconductors are the common material system for the PICs used in optical fiber communications, since they uniquely allow the active and passive devices operating in the most important wavelength windows around 1555 nm and 1310 nm to be combined onto the same InP substrate.
The deeper penetration of optical fiber into the access networks has arisen as optical technologies offer significant advantages in both reach and bit rate over copper-based alternatives and can sustain the projected user driven consumption of bandwidth intensive applications. Optical technologies are therefore expected to dominate in future broadband access networks, and amongst the variety of optical access network architectures under consideration and development, the passive optical network (PON) appears to be the most appealing in terms of cost-effectiveness, bandwidth provisioning, and scalability. In PON networks such as fiber-to-the home (FTTH), fiber-to-the-curb (FTTC), and fiber-to-the-node (FTTN) networks, optical transceivers form the key element of the optical network terminals (ONTs) that terminate either every subscriber node as in FTTH, or small numbers of subscribers as evident in commercial deployments of FTTN within North America. Accordingly, millions of optical transceivers will be needed to complete future FTTH deployments.
The optical transceivers in such ONTs are the interface between the electrical and optical domains and are bi-directional devices that use different wavelengths to transmit and receive signals between the central office (CO) and the ONTs. Owing to the cost sensitive nature of FTTH/FTTN deployments, the directly modulated semiconductor laser is used exclusively as the transmitter technology; and as link specifications become more exacting, the use of single mode distributed feedback (DFB) lasers becomes increasingly important.
Presently, FTTH/FTTN transceivers are built from discrete optoelectronic components that are co-packaged, and it is unlikely that this approach will achieve the cost point, even in volume manufacture, that is required for massive FTTH deployment. To overcome the limitations of hybrid integration, photonic integrated circuits (PICs), in which multiple elements of common or different functionalities are monolithically integrated onto one chip, have been proposed as an attractive solution to mass production: PICs offer the ability to dramatically reduce the component footprint, avoid multiple packaging issues, and eliminate multiple optical alignments. Whilst many aspects of the invention will be described by reference to FTTH/FTTN applications the invention is applicable to distributed feedback lasers designed for other optical telecommunications networks and applications.
In the context of optical networks, PICs are generally designed to operate in the most important wavelength windows about 1555 nm and 1310 nm and in this respect, indium phosphide (InP) and related III-V semiconductors for a suitable material system as these compounds allow for the integration of both active and passive devices operating at these wavelengths. It is also important for the development of PICs destined for such low cost applications as FTTH that the fabrication and manufacturing processes should be ones of low complexity and high yield. This requirement is particularly difficult to meet when a conventional DFB laser is integrated onto the PIC because of the need, within the prior art, for at least one crystalline material re-growth during the fabrication process.
A conventional DFB laser incorporates a periodic perturbation of the propagation medium which causes wavelength selective coupling between the waves propagating bi-directionally along the grating axis by the principle of Bragg reflection. This grating is usually formed by an etched corrugation in a layer that is close to the waveguide core as this leads to strong coupling and favorable laser characteristics. Unfortunately, however, the epitaxial growth that is needed to bury the etched corrugation is very demanding in respect to the quality of the interface between the etched surface and the over-grown epitaxial material. As a consequence, the conventional method of DFB fabrication is neither low complexity nor high yield and therefore not suitable for PICs used in low cost applications.
An alternative approach to DFB laser grating fabrication, which eliminates the low-yield epitaxial re-growth processes, and therefore much more appropriate to PICs, is the so-called laterally coupled (LC) optical grating. In this technique, a grating is formed on either side of the laser ridge waveguide by surface etching semiconductor materials or by selectively depositing metal. Of these two options, the most elegant solution from the perspectives of fabrication simplicity and performance for the LC grating design is referred to within the prior art as an effective-ridge laterally coupled surface etched grating (LC-SEG). Within the LC-SEG the lateral optical confinement of the ridge waveguide used for the laser is provided by and combined with the optical Bragg grating, being formed thereby from two sets of narrow trenches etched from the top surface of the ridge, along the propagation direction and at a fixed distance from one to the other.
The first experimental demonstration of a DFB laser using LC-SEG was reported by L. M. Miller et al, “A Distributed Feedback Ridge Waveguide Quantum Well Heterostructure Laser” [Technology Lett., Vol. 3, No 1, PP. 6-8, 1991]. Here, direct write electron beam lithography and reactive ion etching (RIE) were used to fabricate third- and fifth-order gratings in the InGaAs—GaAs—AlGaAs material system to demonstrate lasing at 1.05 μm. Subsequently, the effective-ridge LC-SEG was extended to DFB lasers at longer wavelengths in other material systems, for example, in GaInAsP—InP, see for example H. Abe et al, “1.55 μm Surface Grating Strained MQW-DFB Laser” [Ext. Abstr., 58th Annual Meet. Jpn. Soc. Applied Physics, P. 1111, 1997]; Y. Watanabe et al, “Laterally Coupled Strained MQW Ridge. Waveguide Distributed-Feedback Laser Diode Fabricated by Wet-Dry Hybrid Etching Process” [IEEE Photon. Technology Lett., Vol. 10, No. 12, pp. 1688-1690, 1998] and Watanabe et al [U.S. Pat. No. 6,714,571]. The approach also being extended to AlGaInAs—InP material systems, see for example J. Wang et al, “1.55-μm AlGaInAs—InP Laterally Coupled Distributed Feedback Laser” [IEEE Photon. Technology Lett., No. 7, pp. 1372-1374, 2005].
In another example of the previous art, a first order surface grating of chromium, rather than etched semiconductor materials, was deposited by electron beam lithography. See Schreiner et al, “Laterally gain-coupled 1.57 um DFB lasers with chromium surface grating and self-aligned Ti/Pt/Au ohmic contact” [Electron. Lett., 36, PP 636-637, 2000].
Although it is evident from such prior art that LC gratings, and in particular LC-SEG structures, can be integrated with single growth PICs, a major disadvantage of this type of grating in relation to LC-DFB lasers is that the propagating optical mode only interacts with the surface grating through its evanescent field; and so the coupling is much weaker than can be achieved with a buried grating. As a result LC-DFB lasers must have a longer cavity to compensate for the lower coupling; and in direct modulation applications, this is an issue because of the parasitic capacitance of laser, which limits the bandwidth, will therefore is reduced through the increased length of the LC-SEG based LC-DFB lasers.
Accordingly, prior art approaches have sought to maximize the coupling coefficient in LC-DFB laser designs by using low order gratings which require slow and expensive processes such as direct write electron beam lithography that are not suited to low cost PICs. While other prior art, such as Reid et al, “Narrow Linewidth and High Power Distributed Feedback Lasers Fabricated without a Regrowth Step” [Proceed. of European Conference on Optical Communications 2003 (ECOC 2003), Rimini, 2003] describe the fabrication of a LC-SEG DFB laser using processes that are very desirable for low cost PICs, namely commercial optical stepper lithography and inductively coupled plasma (ICP) etching systems; the laser described therein being designed for a high power application where a low coupling coefficient and long laser cavity were acceptable.
It would be advantageous therefore to provide a solution removing the constraints of the prior art, by offering increased design, fabrication and utilization opportunities for the approach within integrated photonics components, namely PICs. It would be further advantageous if the solution was compatible to standard semiconductor materials, exploited an epitaxial semiconductor structure growth approach using a single growth step, and supported a plurality of active waveguides, each active waveguide for operating upon different operating wavelengths with bandwidth commensurate to the application.